Increasing energy demands, along with the need to use energy from intermittent, renewable resources has led to a continued interest in the field of energy storage and delivery, including the field of batteries. Such energy devices must safely deliver required energy output for many uses, including transportation and portable power systems. Lithium-ion and lithium metal batteries have emerged as one candidate to meet such demands.
The term “lithium-ion battery” or “lithium-ion cell” encompasses batteries where the anode and cathode materials act as a host for the lithium ion (Li+) and the terms are used equivalently and interchangeably throughout this disclosure. Lithium ions migrate from the anode to the cathode during discharge and are inserted into the voids in the crystallographic structure of the cathode. The ions reverse direction during charging. Alternating layers of anode and cathode are separated by a porous film, known as a separator. An electrolyte, typically comprising an organic solvent along with dissolved lithium salt provides the media for the lithium ion transport. Lithium-ion cells can be made by stacking alternating layers of electrodes, or by winding long strips of electrodes into a roll configuration (typical for cylindrical cells). Electrode stacks or welds are typically inserted into hard cases that are sealed with gaskets, laser-welded hard cases, or enclosed in foil pouches and heat-sealed. Lithium batteries are similar in principle to lithium-ion batteries, but are often single-use primary batteries with lithium metal anodes that offer high energy density. For purposes of this application, lithium metal batteries are included in the general category of “lithium ion” batteries, and the advantages of the present disclosure are understood to apply also to lithium metal batteries. It is further understood, for the purposes of this application, that the terms “lithium ion cell” and “lithium ion battery” are used interchangeably, with equivalent meaning.
Lithium-ion cells in today's market therefore have similar designs featuring a negative electrode (anode) made from carbon/graphite coated onto a copper current collector, a metal oxide positive electrode (cathode) coated onto an aluminum current collector, a polymeric separator, and an electrolyte comprising a lithium salt in an organic solvent.
Lithium-ion cells have a safe operating voltage range over which it can be cycled that is determined by specific cell chemistry. The safe operating range is a range where the cell electrodes will not rapidly degrade due to lithium plating onto the electrodes, will not undergo copper dissolution, or sustain other undesirable reactions, including thermal runaway, for example.
Typical electrolytes feature a mixture of organic carbonates such as ethylene carbonate or diethyl carbonate. The mixture ratios vary depending on the desired cell properties. The electrolytes comprise lithium-ions provided to the electrolyte in the form of lithium salts, such as, for example, lithium hexafluorophosphate (LiPF6).
Typical lithium-ion cells may also contain various additives to increase the performance, stability and safety, such as, for example, additives that act as flame retardants, overcharge protectors, cathode protection agents, lithium salt stabilizers or corrosion inhibitors.
During normal device operation, the temperature of lithium ion cells increases as a result of the exothermic lithium reactions, and cells are designed to efficiently transport the heat to the outside of the cell. Problems within the cell can cause the internal temperature to increase beyond an acceptable limit, which causes the lithium reaction rates to increase, further increasing the temperature of the cell. Most lithium-ion cells are not designed to withstand temperatures above approximately 60° C. during operation or storage. Most commercial lithium-ion cell contain chemistries resulting in breakdown occurring in the temperature range of from about 75° C. to about 90° C., during the time discharge rates are high.
Thermal runaway refers to cell conditions where rapid self-heating of the cell occurs due to the inherent exothermic chemical reaction of the highly oxidized cathode and the highly reducing anode. In a thermal runaway reaction, a cell rapidly releases stored energy. Since lithium-ion cells have high energy densities and flammable electrolytes, lithium-ion cell thermal runaway can be dangerous, and will at least lead to the destruction of the cell, and could contribute to conditions that could lead to the destruction of the device being powered. Certainly, when cells are arranged in series to discharge power, thermal runaway in one cell can damage adjacently-positioned cells.
Attempts to achieve lithium-ion cell shutdown (as a safety measure) in the event of thermal runaway have focused on separator technology or the use of solid-state electrolytes. Lithium-ion cell separators are typically made from porous polymers including polyethylene, polypropylene, or combinations thereof. The separators act to prevent the direct contact between the anode and cathode. The pores in the separators allow the cyclical transport of lithium ions during alternating charge and discharge cycles via diffusion through the separator. When temperatures elevate above a predetermined “safe” operating temperature, many separators are designed to soften, thus closing the pores, and stopping lithium ion transport through the separator. While such a shutdown of the cell can inhibit the threat of thermal runaway from occurring, the shutdown permanently disables the cell in the case of an internal temperature rise. In addition, at certain internal temperatures, the separators may melt entirely, which cannot only lead to permanent cell failure, but also risks fire or explosion. Similarly, the use of solid state electrolytes has been proposed, where low ionic conductivity limits cell performance (particularly the power density of the cell). However, like the proposed separators, thermal decay occurs at elevated temperatures. While this potentially improves safety by reducing the chance of fire or explosion, the mechanisms triggering such safety measures render the cells useless.
Managing heat generation in small format lithium-ion cells has become relatively routine due to the cells' limited size. This has resulted in limited risk of thermal failure. Therefore, some commercially available cells adopt arrays of small format cells. However, large format lithium-ion cells have shown an increased risk of catastrophic thermal failure due to, for example, high surface contact between electrode and electrolyte, high amount of heat generated in a confined space, and large distances to dissipate heat from the cell. Further, limitations in thermal management have limited the size and scale of secondary lithium-ion batteries.
Methods and apparatuses for lithium-ion cells that could ameliorate or impede internal temperature rises, and reduce the hazards of thermal runaway would be highly advantageous. Further, methods for impeding thermal runaway while also preserving the function of the lithium-ion cell itself would be also highly advantageous.